Despite what native speakers may tell you, whether a word is spelled with b or v has no effect on its pronunciation in spontaneous speech. This means that a ver and haber are pronounced identically. This is why many native speakers mix up b and v when writing.
One phoneme: /b/
The technical way to say this is that Spanish has a single phoneme, /b/, that corresponds without distinction to both written b and v. The only exceptions I have heard are in prepared speech, for example, from radio announcers, actors or singers who have been taught to make a distinction between b and v as part of their professional training.
I did once hear a native speaker trying to observe a distinction between b and v when presenting in front of a group. It came across as an affectation, perhaps the result of a misguided attempt to impress the listeners with her knowledge of French or English. She was unable to sustain the effort in a consistent way.
Two allophones: [b], [β]
Nevertheless, there are in fact two distinct sounds that can and should be produced for b/v. The choice is determined entirely by the sound that precedes the b/v. It is not up to the speaker’s preferences. The so-called plosive variant, represented by the symbol [b] as in English, is used for b/v after a pause or a nasal consonant. The fricative variant, represented by the symbol [β], is used for b/v elsewhere.
|vaso (drinking glass)||[ˈba so]||voiced bilabial plosive [b]|
|IVA (sales tax)|
iba (go, imperfect)
|[ˈi βa]||voiced bilabial fricative [β]|
We've moved, Mexico City
Let’s look first at the plosive sound, [b]. This is basically the same sound that is associated with the letter b in English1. In Spanish it should be used when b/v appears at the beginning of a phrase (an intonation group), for example, after a pause or at the beginning of an utterance. For this reason, the first sound of ¡Ven! is [b].
This plosive sound should also be used after the nasal consonants m and n. So we have the sound [b] in ambos and en vano (in vain). Note that in the latter example, it is normal for the n to be pronounced [m], due to the [b] that follows it. Try this and you’ll see how natural it is is produce an [m] before a [b], since they have the same place of articulation: both lips.
Now let’s talk about the second sound for b/v, the fricative sound, [β]. This sound is probably not the first sound you learned for b/v in your Spanish class, if indeed you learned it at all. However, it is the first b/v sound mastered by native speakers as children. The sound is not the same as either [b] or [v], so it may be a little tricky at first to produce it or to even perceive it as a b/v sound. It is used in Spanish for b/v everywhere except for the contexts we just mentioned above for the plosive consonant [b], namely after a pause or a nasal consonant.
Let’s start by examining how the English v is produced. The lower lip approaches the upper teeth, touching them in places but not producing complete closure between the lips and teeth. Air under pressure can still escape between in the gaps between the lower lip and the upper teeth, causing the lower lip to vibrate. Make a longer-than-usual English v sound, [vvvvvvvv] and see if you feel this vibration.
To produce the Spanish b/v, start with the English v as your model, but instead of approaching the upper teeth, approach the upper lip, making partial contact between the lower and upper lips.
I first learned to produce this sound by positioning the lower lip very, very close to the upper lip, allowing air to pass only through the narrow horizontal slot you have created between the lips. You should feel a vibration in your lips. If you’re doing it right, your teeth are not involved at all. You should be able to extend the b/v sound indefinitely: [aββββββββββββββa].
Nowadays I sometimes produce this sound using a different technique. I seal my lips lightly where they meet at their center and let the air under pressure escape out the sides. The auditory effect produced is the same. In some situations I find this easier to control.
Practicing the Contrast
Now practice by contrasting both sounds, [b] and [β]. For example, bebe (take a drink) [ˈbeβe] or bebé (baby), [beˈβe].
Next, alternate sounds to constrast versions that sound like Spanish with versions that sound like English: uva (grape): [uβa], [uba], [uva]. Only the first one sounds like Spanish.
How important is it to produce b/v correctly?
Although native speakers — including many teachers of Spanish for Foreigners — are not generally consciously aware of these two b/v sounds in Spanish, there’s no question that using an English b or v between vowels in Spanish will contribute to the foreigness of your accent. So will using an English b or v in non-intervocalic contexts, for example, as the first sound of a phrase.
You’ll also need to open your ears and recognize the sounds that native speakers are producing for b/v rather than the ones you think you hear. You might be surprised at what you hear. With time and practice, your efforts will pay off.
Other Spanish consonants with fricative and plosive variants
The Spanish /d/ uses similar variants, a fricative one ([ð]) used intervocalically and a plosive one ([d]) used in other contexts. The consonant /ɡ/ undergoes the same variation: fricative [ɣ] and plosive [ɡ].
1 The Spanish [b] is voiced for longer than its English counterpart. English can get away with a shorter period of voicing (that is, a negative voice-onset time) due to [p], the voiceless counterpart, being distinguished from [b] via aspiration.